Wood and wood products have been used throughout history. It is recognised that on exposure to the environment untreated wood undergoes various undesired reactions, e.g. fungi and bacteria can cause oxidation, hydrolysis and reduction of the chemical components of wood and wood products, i.e. degradation of wood surface and cell wall polymers (cellulose, hemicelluloses and lignin), in chemical and enzymatic processes, affecting the properties of the wood.
There are a number of prior art methods for treating wood and wood-based products in order to increase resistance to undesired reactions. These methods typically involve contacting the surface of the wood or wood-based product with an agent comprising toxic or corrosive chemicals, of which there are several commercially available e.g. chromated copper arsenate (CCA), alkaline copper quaternary (ACQ) etc.
These prior art methods have the disadvantages that although they are effective methods for the provision of resistance to undesired reactions, their chemical properties give rise to environmental concerns in the case of e.g. CCA, and are extremely corrosive in the case of e.g. ACQ.
More recent technological advances provide the chemical modification of wood. The most studied of these methods is the reaction of wood with acetic anhydride. Wood contains an abundance of free hydroxyl groups that readily absorb and release water depending on the conditions in which the wood finds itself. When reacted with acetic anhydride the free hydroxyl groups are substituted with acetyl groups (reaction scheme I).WOOD-OH+CH3—C(═O)—O—C(C═O)—CH3→WOOD-O—C(═O)—CH3+CH3C(═O)—OH  I
In order to achieve even acetylation the wood must be in a partially swollen state, i.e. moist. This is disadvantageous in that water molecules in the wood will react with acetic anhydride to give acetic acid thus resulting in loss of reagent. Optimum conditions for acetylation are at temperatures in excess of 100° C. as is disclosed in U.S. Pat. No. 4,804,384. It is well known in the art that the mechanical properties of wood suffer as temperature rises, i.e. increasing temperature has a detrimental effect on mechanical properties such as modulus of elasticity, shear modulus, bending strength, tensile strength, compressive strength and shear strength see, e.g. Green et al. 2003.
Further methods are disclosed in which chemical modification is achieved using phosphorus compounds or using sterically hindered amines, e.g. in US patent application 2011/108,782, U.S. Pat. Nos. 4,692,355 and 6,653,324.